1. Field of the Invention
The invention relates to pistons for reciprocating machines employing a compression of a gaseous fluid, such as internal or external combustion engines and compressors, i.e. for machines in which the working gas is at both high pressure and high temperature.
2. Field of the Invention
The engines, these engines may be of various types, such as compression ignition or controlled ignition engines; two-stroke engines (in particular effecting a scavenging through a valve), four-stroke or other engines; engines having a free piston or a piston connected to a crankshaft or other mechanism; engines having a pressurized or non-pressurized crankcase; in the case of connected piston, the latter may be pivotally connected to a connecting rod directly or indirectly (by a cross-head), the pivotal connection being for example cylindrical (by means of a piston pin which is or is not journalled in a boss) or spherical.
In these machines, the seal between the working chamber and the chamber located on the other side of the piston (for example the crankshaft case) is usually achieved by metal rings whose outer surface intimately fits the inside wall of the cylinder so as to constitute a lubricated contact and thereby avoid seizing and limit wear.
The difficulties of this solution are the following:
(1) It is not possible to avoid transient differences of temperature between the inner wall of the cylinder, the ring and the piston, of several tens of degrees (centigrade) which result in differential expansions of several tenths of a millimeter, much greater than the thickness of the film of oil which is measured in microns. There are added to the differences in diameter due to the differential thermal expansion, when the cylinders are lined, those due to the expansion of the relatively thin liner of the cylinder under the effect of the pressure of the working gas. This is why it is necessary, as is known, to split the ring (except in exceptional cases such as mentioned in FR-A-707.660 which will be discussed hereinafter) and allow it freedom of movement in the groove of the piston, the outside diameter of the latter being always less than the inside diameter of the cylinder.
(2) In the neighborhood of the top dead center (the combustion top dead center in the case of internal combustion engines) where the pressure and temperature are maximum in the working chamber, the velocity of the ring and the hydrodynamic supporting qualities of the film of oil are low. The ring is then applied against the inner wall of the cylinder and the residual oil is then urged upwardly (if it is assumed conventionally that the working chamber is located above the piston) and downwardly to tee point of resulting in metal-to-metal contact which causes wear of the upper end of the cylinder and ring. The best solution for retarding the disappearance of the film of oil would be to increase the height of the ring and therefore the volume of oil trapped for an identical escape section, but this is difficult to achieve owing to the cyclic tilting undergone by the piston around its axis of connection with the connecting rod and which leads to giving the ring a convex curved shape so as to maintain a circular contact on the inner wall of the cylinder and a good bearing of the ring on the lower side of the groove of the piston. The drawback of this solution is that it results in specific contact pressures which may reach several hundreds of bars, bearing in mind the pressure-sealed operation of the ring. One is therefore obliged to pass through the top dead center with a more or less limited unctuous lubrication condition of operation, the oil being maintained on the inner wall of the cylinder in grooves which are usually helical and provided by a treatment of the wall currently termed "honing".
Furthermore, the splitting of the ring allows a passage of burning hot gases along the piston (phenomenon termed "blow-by"), resulting in pollution of the lubricating oil and heating of the piston-cylinder interface; further, it results in a more or less rapid wear of the top of the cylinder owing to the disappearance of the film of oil and the introduction of abrasive particles. It must be added that, in order to be free in its groove, the metal ring beats at a certain moment of the cycle, i.e. passes form a bearing against the lower side of the groove to a bearing against the upper side of the latter. It then liberates a gas passage through the inner end of the groove which entrains the oil toward the combustion chamber when the pressure between the rings is higher than the pressure in this chamber, which increases the oil consumption.
It is known that a piston performs different functions:
(a) transmission to a connecting rod of the force due to the thrust of the gases which is exerted either solely on the upper side of the piston, or also on its lower side (double-acting pistons or pistons of engines having a pressurized crankcase) in the case of internal combustion engines, or transmission in the opposite direction in the case of compressors;
(b) guiding of the piston in the cylinder (or the liner of the latter) in which it moves in accordance with a quasi-rectilinear reciprocating motion and the taking of the lateral reaction of the cylinder (or liner);
(c) sealing between the chambers located on each side of the piston in which necessarily different pressures prevail;
(d) limitation of the oil flow from one side of the piston to the other.
Owing to mechanical frictions associated with the sliding of the piston in the cylinder, the transfer of heat between the gases and the wall of the cylinder, due to the heating of the gases during the compression or the combustion, and the usually intense cooling of the cylinder, a temperature difference .DELTA.T occurs between the piston and the liner.
This difference .DELTA.T is variable:
according to the position of the considered point
piston top, piston bottom; PA1 situation with respect to the boss of the piston; PA1 according to the conditions of operation of the machine: PA1 stoppage; PA1 cold or hot; PA1 according to the speed and/or the load of the machine. PA1 barrel profile of the piston PA1 oval section (due to the boss), PA1 an intense heating of the top of the piston and the cylinder; PA1 pollution and oxidation of the oil; PA1 the entraining of abrasive particles of carbon or soot or ash resulting in scratching on the cylinder (or the liner) and the outer surface of the ring; PA1 the gumming--by destruction of the oil--of the ring in its groove resulting in its immobilization and increasing the sealing defect and eventually causing the seizing of the piston in the cylinder. PA1 x being the height of the considered cylinder section and PA1 P being the pressure at which the ring bears against the cylinder. PA1 .sigma. being the compression stress, PA1 E being the Young's modulus of the material of the ring (namely about 22,000 hbars for steel).
Therefore, an operational clearance is usually provided between the piston and the cylinder in order to avoid or limit any gripping of the piston in the cylinder upon differential thermal expansions.
This operational clearance is obtained in a practically uniform manner by giving to the piston a complex shape in the cold state:
which increases the complexity and the cost of the piston and precludes an intimate contact between the piston and the cylinder (and consequently an undesirable concentration of the contact pressure).
The existence of this clearance on one hand results in a defect in the guiding which results in non-rectilinear movements of the piston (tilting, whipping, etc.). For example, when the piston is directly pivotally connected to a connecting rod, the angulation of the latter results in a cyclic tilting of the piston. The existence of this clearance on the other hand results in intense noise, shocks, wear and fatigue.
These non-rectilinear movements of the piston require locating the piston rings close to the upper side of the piston in order to avoid contact of the head of the piston with the cylinder (or liner). Consequently, the piston ring means are located in the hottest part of the piston and the lubricated path of the cylinder is less protected from the hot gases.
These non-rectilinear movements of the piston also require a convex curved profile on the rings in order to maintain the continuity of the circle of contact between the ring and the cylinder. This results in an increase in the specific pressure of contact between the rings and the cylinder.
Around the top dead center (TDC) where the sliding velocity of the piston is insufficient to maintain a hydrodynamic pressure in the film of oil, while the bearing pressure of the ring is maximum due to the increase in the pressure of the gases the thickness of the protective film of oil becomes too small or nil and results in wear of the cylinder (or of the liner) and wear of the ring.
The required movement of the ring in its groove results in the floating of the ring when it changes its bearing against one side of the groove to the other, resulting in movements of gas and oil.
The splitting of the rings required for adapting at each instant the perimeter of tee ring to that of the cylinder, also results in movements of gas and oil.
In particular, and as recalled hereinbefore, a very harmful gaseous flow (blow-by) occurs between the upper side and the lower side--adjacent to the crankcase--of the piston. This flow results in: